IEEE 802.11
A technical Overview
Pablo Brenner
Director of Engineering
BreezeCom
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Introduction
Overview of the Emerging 802.11 Standard
Technical details
Special areas of interest?
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Why do we need a Standard?
Multi-Vendors Compatibility
Protects customers investment
High Volumes reduce prices
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Why Not Just use Wireless
Ethernet?
First Ethernet predecessor was Radio-Based
(ALOHA)
Ethernet is simple, cheap, widely accepted…
But...
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Ethernet could not do it.
Collision Detection
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– Would require a Full-Duplex Radio
– Wouldn’t help on recognizing collisions on the receiver
end
Carrier Sense
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– The fact that a station doesn’t sense traffic does not mean
that the receiver side can receive (Hidden Stations)
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Ethernet could not...
The “Mall Problem”
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– How can you partition two collocated networks?
Mobility and Roaming
Security Issues
Power Saving Requirements
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Wireless LAN System
802.11 C om ponents
BSS 1
STA 1
STA 2
AP
DS
AP
BSS 2
STA 3
STA 4
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802.11 Architecture
MLME
MAC
MAC MIB
MLME_GET/SET
PLME_GET/SET
PLCP
Station
Management
PHY MIB
PLME_GET/SET
PLME
PMD
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Basic Access Mechanism
CSMA/CA
Stations listen before transmission
If medium free for more than DIFS: transmit
If not, use backoff mechanism.
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Immediate access when medium
DIFS
is free >= DIFS
Contention Window
PIFS
DIFS
SIFS
Busy Medium
Backoff-Window
Next Frame
Slot time
Defer Access
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Select Slot and Decrement Backoff as long
as medium is idle
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Other Collision Avoidance
Mechanism: NAV
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Needed to handle Hidden Stations Problem
SA2
AP
SA1
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NAV Operation (RTS/CTS)
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Stations exchange Duration information using
short frames (RTS/CTS), other stations, recognize
the medium as busy for the said duration
G1=SIFS
G3=DIFS
CW=Contention Window
G3
Src
Data
RTS
G1
Dest
G1
G1
CTS
Ack
G3
Other
NAV (RTS)
CW
Next MPDU
NAV (CTS)
Defer Access
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Backoff after Defer
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Collision “detection”
mechanism
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Real Collision detection would require a fullduplex radio (high cost)
Unable to recognize collisions at the
receiving end
Uses Positive Acknowledge (ACK)
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Fragmentation
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Long fragments: higher probability of error
Microwave ovens interference
– (4ms noise, 4 ms clear)
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Collision recovery is less expensive if we use
fragmentation
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Frame format
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FC
Duration
ID
Addr 1
Addr2
Addr 3
Seq
Nr
2
2
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6
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Addr 4
6
Data
CRC
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Frame Control Field
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Protocol Version (2 bits) = 0
Type (6 bits)
ToDS
FromDS
More Frag
Retry
Power Mgt
More Data
WEP
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Beacon Frames
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Send Periodically by the AP
Provide information for new stations
Keeps Synchronization
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Init Procedure
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Scanning
– Passive
– Active
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Authentication
Association
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Roaming Procedure
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Not defined in the standard, left as
implementors decision
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BreezeCom Patented
Algorithm
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Designed for Rotterdam Port requirements.
The AP sends periodic “Neighbor Beacons”
when appearing as a regular station
Stations receive Synch information, so learn
how and when to join the new AP
Stations compare Signals Strength, and make
the decision
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BreezeCom Algorithm (ctd...)
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Station knows which AP is it roaming to so
lets both APs (new and old) know about that.
New AP starts accepting frames addressed to
the roaming station, buffers stations as
regular “Power Saving” mode
Old AP inserts “Jump Message” for synch
the actual roaming.
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BreezeCom Algorithm (ctd...)
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Stations waits for “Jump” message, and
roams when received.
Station Associates with new AP, and then
receives buffered packets.
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How do APs know about
Neighbors?
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Automatically, first station that roams “the
hard way” lets AP Know about the
neighboring relationship
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Load Balancing
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Currently no provisions on 802.11
Need a “Load” parameter in beacon frames
Needs special BreezeCom SNAP formatted
beacon (same format as Neighbor Beacon)
BreezeCom will disclose both algorithms to
PCMCIA “partners”
Use of Exponential Random Backoff
procedure to prevent stations from bouncing
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Load Balancing (ctd...)
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AP may use the “Jump” frame to cause a
specific station to join a different AP.
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Power Saving Mechanisms
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Power Saving Stations Notify the AP
AP buffers frames for Power Saving Stations,
and broadcast/multicast frames
AP sends TIM (Traffic Information Map) on
Beacon Frames
Some Beacons contain DTIM (Delivery
TIM)
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Power Saving Mech (ctd...)
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Power Saving Stations may poll the AP for
the buffered frames using PS-POLL frames
The AP will send mcast/bcast frames
immediately after the DTIM
The AP will send unicast buffered frames
immediately after the mcast/bcast frames
after the DTIM.
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802.11 Status
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Draft 5.0 in Sponsor Ballot
Approval expected Q1 97.
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802.11 Status (ctd...)
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First interoperability tests performed with 4
PCMCIA vendors
Results very promising, beyond expectations
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BreezeCom @ 802.11
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4 Voting members
2 Highly Contributing members
– Naftali Chayat (BreezeCom Chief Scientist)
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Author of the 2 MBit/s FH original proposal
Presented 3 MBit/s proposal
Chairman of the “FH Higher Rates interest group”
– Pablo Brenner (BreezeCom Director of Engineering)
» Author of several proposals (MultiRate Support)
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